MAPK Signalling and the Role of ERK1/2
The extracellular signal regulated kinases (ERK1/2) are ubiquitously expressed protein serine/threonine kinases that comprise a key component of the mitogen-activated protein kinase (MAPK) signalling pathway. The MAPK pathway is an evolutionary conserved cell signalling pathway that regulates a variety of cellular processes including cell cycle progression, cell migration, cell survival, differentiation, metabolism, proliferation and transcription. The ERK/MAPK signalling pathway responds to the extracellular stimulation of cell-surface receptor tyrosine kinases (RTKs). Upon activation of RTKs, the RAS GTPases (K-RAS, N-RAS and H-RAS) are converted from an inactive GDP-bound state to an active GTP-bound state. Activated RAS phosphorylates and thereby activates RAF (A-RAF, B-RAF and C—RAF), which in turn phosphorylates and activates the dual-specificity kinase MEK (MEK1/2). Subsequently, activated MEK phosphorylates and activates ERK1/2. Upon activation, ERK1/2 activates multiple nuclear and cytoplasmic substrates. There are currently >200 known ERK1/2 substrates, which include transcription factors, kinases, phosphatases and cytoskeletal proteins (Roskoski, Pharmacol. Res. 2012; 66: 105-143).
A number of isozymes of ERK have been identified (ERK1, ERK2, ERK3/4, ERK5, ERK7) but the two most widely studied isozymes are ERK1 and ERK2: see R. Roberts, J. Exp. Pharm., The extracellular signal-regulated kinase (ERK) pathway: a potential therapeutic target in hypertension, 2012: 4, 77-83, and Cargnello et al., Microbiol. & Mol. Biol. Rev., Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases 2011, 50-83.
Upregulation of ERK1/2 Signalling in Cancer
ERK1/2 activity is commonly upregulated in cancer, as a result of activating mutations within upstream components of the MAPK pathway. Approximately 30% of human cancers contain activating RAS mutations (Roberts and Der, Oncogene. 2007; 26: 3291-3310). K-RAS is the most frequently mutated isoform and is mutated in 22% of all tumours. KRAS mutations are particularly prevalent in pancreatic adenocarcinoma (70-90%), non-small cell carcinoma (10-20%) and colorectal cancer (25-35%) (Neuzillet et al., 2014. Pharmacol. Ther. 141; 160-171). N-RAS and H-RAS mutations occur in 8% and 3% of cancers, respectively (Prior et al., Cancer Res. 2012; 72 (10); 2457-2467). Notably, activating N-RAS mutations have been reported in 15-20% of melanoma cases. Furthermore, activating B-RAF mutations occur in 8% of all tumours and are particularly prevalent in melanoma (50-60%), papillary thyroid cancer (40-60%), colorectal cancer (5-10%) and non-small cell lung cancer (3-5%) (Neuzillet et al., 2014. Pharmacol. Ther. 141; 160-171). In addition to the occurrence of activating RAS and RAF mutations, the MAPK signalling pathway can also be up-regulated in cancer by the over-expression or mutational activation of upstream RTKS such as EGFR (Lynch et al., N Engl J Med. 2004; 350: 2129-2139), HER2 (Stephens et al., Nature. 2004; 431: 525-526) and FGFR (Ahmed et al, Biochim. Biophys. Acta Mol. Cell. Res. 2012; 1823: 850-860).
There are multiple mechanisms by which aberrant ERK1/2 signalling can contribute to cancer progression. Upon activation, ERK1/2 phosphorylates and activates a wide range of transcription factors that are involved in promoting cell proliferation and differentiation, such as c-Fos (Murphy et al., Nat. Cell Biol. 2002: 4 (8):556-64) and ELK-1 (Gille et al., EMBO J. 1995; 14 (5):951-62). In addition, ERK1/2 signalling is known to promote cell cycle progression via multiple mechanisms, including the induction of D-type cyclins and repression of the cyclin-dependent kinase inhibitor p27KIP1 (Kawada et al., Oncogene. 1997; 15: 629-637, Lavoie et al., J. Biol. Chem. 1996; 271: 20608-20616). Furthermore, ERK1/2 signalling can promote cell survival by regulating a range of apoptotic proteins. Examples of such mechanisms include the ERK1/2-dependent repression of the pro-apoptotic BCL-2 family proteins BIM1 and BAD (She et al., J. Biol Chem. 2002; 277: 24039-24048. Ley et al., J. Biol. Chem. 2003; 278: 18811-18816) and the ERK1/2-dependent stabilisation of anti-apoptotic proteins such as MCL-1 (Domina et al., Oncogene. 2004; 23: 5301-5315).
Role of ERK1/2 in MAPK Inhibitor Resistance
A wide range of pre-clinical studies have demonstrated that the inhibition of the MAPK pathway suppresses the growth of cancer cell lines harbouring B-Raf or Ras mutations (Friday & Adjei, Clin. Cancer Res. 2008; 14: 342-346). The RAF inhibitors vemurafenib and dabrafenib, and the MEK inhibitor trametinib are clinically approved for the treatment of BRAF-mutant melanoma. These agents elicit profound anti-tumour responses in the majority of patients, although the duration of response is short-lived, due to the onset of acquired drug resistance (Chapman et al., N. Engl. J. Med. 2011; 364 2507-2516. Hauschild et al., Lancet. 2012; 380: 358-365. Solit and Rosen, N Engl J Med. 2011; 364 (8): 772-774. Flaherty et al., N. Engl. J. Med. 2012; 367: 1694-1703). Multiple mechanisms of acquired B-RAF inhibitor resistance have been identified. These include the upregulation or activation of alternative MEK activators such as C—RAF or COT1 (Villanueva et al, Cancer Cell. 2010; 18:683-95. Johannessen et al., Nature. 2010; 468: 968-72); the upregulation of RTK or NRAS signalling (Nazarian et al.; Nature. 2010; 468:973-7), and the onset of MEK activating mutations (Wagle et al., J Clin Oncol. 2011; 29:3085-96). Mechanisms of MEK inhibitor-resistance include the occurrence of MEK mutations that reduce drug binding or enhance intrinsic MEK activity (Emery et al., Proc Natl. Acad. Sci. 2009; 106: 20411-20416. Wang et al., Cancer Res. 2011; 71: 5535-5545), and BRAF or KRAS amplification (Little et al., Biochem Soc. Trans. 2012; 40(1): 73-8). A common feature of RAF or MEK inhibitor resistance mechanisms is the re-activation of ERK1/2 signalling, which drives proliferation and survival of the cells in the presence of inhibitors. Based on this observation, it has been suggested that direct ERK1/2 inhibition may be an effective therapeutic approach to overcoming acquired RAF or MEK inhibitor resistance. There is pre-clinical evidence that the inhibition of ERK1/2 overcomes acquired RAF or MEK inhibitor resistance (Hatzivassiliou et al., Mol Cancer Ther. 2012; 11(5):1143-54. Morris et al., Cancer Discov. 2013; 3 (7):742-50)
Additional Diseases
In addition to oncology, abnormal ERK1/2 signalling has also been reported in other diseases including cardiovascular disease (Muslin, Clin. Sci. 2008; 115: 203-218), Alzheimer's disease (Giovannini et al., Neuroscience. 2008; 153: 618-633), polycystic kidney disease (Omori et al., J Am Soc Nephrol. 2006; 17:1604-1614), Asthma (Duan et al., J Immunol. 2004; 172: 7053-7059) and emphysema (Mercer et al., J. Biol. Chem. 2004; 279: 17690-17696).